1. Field of the Invention
The present invention relates to a method and related apparatus for improving Multi-input Multi-Output (MIMO) operation for a user equipment in a wireless communications system, and more particularly, to a method and related apparatus for improving MIMO operation according to a configuration of a Hybrid Automatic Repeat Request (HARQ) process identifier corresponding to a secondary transport block.
2. Description of the Prior Art
The third generation (3G) mobile telecommunications system has adopted a Wideband Code Division Multiple Access (WCDMA) wireless air interface access method for a cellular network. WCDMA provides high frequency spectrum utilization, universal coverage, and high quality, high-speed multimedia data transmission. The WCDMA method also meets all kinds of QoS requirements simultaneously, providing diverse, flexible, two-way transmission services and better communication quality to reduce transmission interruption rates. Through the 3G mobile telecommunications system, a user can utilize a wireless communications device, such as a mobile phone, to realize real-time video communications, conference calls, real-time games, online music broadcasts, and email sending/receiving. However, these functions rely on fast, instantaneous transmission. Thus, targeting the third generation mobile telecommunication technology, the prior art provides High Speed Package Access (HSPA) technology, which includes High Speed Downlink Package Access (HSDPA) and High Speed Uplink Package Access (HSUPA), to increase bandwidth utility rate and package data processing efficiency to improve uplink/downlink transmission rate.
HSDPA and HSUPA adopt Hybrid Automatic Repeat Request (HARQ) technology to enhance retransmission rate and reduce transmission delay. HARQ is a technology combining Feed-forward Error Correction (FEC) and ARQ methods, which makes a Universal Terrestrial Radio Access Network (UTRAN) trigger retransmission according to positive/negative acknowledgement signals (ACK/NACK) transmitted from a user equipment (UE), and the UE stores useful information about the last failed transmission for later use.
To further increase the downlink data rate, 3GPP introduces a Multi-input Multi-Output (MIMO) technology, with which a UE and a base station, known as a Node-B, utilize multiple antennas to transmit/receive radio signals. The MIMO technology can be further integrated with spatial multiplexing, beam forming and spatial diversity technologies to reduce signal interference and increase channel capacity. To control the MIMO operation of the UE, the UTRAN can set configuration of MIMO operation in radio resource control (RRC) messages with information elements (IEs) in a RRC protocol layer, and send the RRC messages to the UE through corresponding RRC procedures. Correspondingly, the UE uses a MIMO_STATUS variable for storing received parameters. When the MIMO_STATUS variable is set to “TRUE”, the MIMO operation is configured.
When the UE is not configured in the MIMO operation, the UE can only receive one transport block on a High Speed Physical Downlink Shared Channel (HS-PDSCH) in one Transmission Time Interval (TTI). A HARQ procedure for the UE includes 8 HARQ processes at most and each of the HARQ processes is configured to a HARQ process identifier, which is represented by an integer between 0 and 7. The UE partitions the soft memory buffer according to the HARQ process identifier. On the other hand, when the UE is configured in the MIMO operation, the UE can receive two transport blocks on the HS-PDSCH in one TTI, and a number of the HARQ processes of the HARQ procedure increases to 16 at most. The number of the HARQ processes, Nproc, is defined in an IE “Number of Process”. After the UE receives an IE “Number of Process”, the UE assigns all of the HARQ process identifiers of the Nproc HARQ processes for 0 to (Nproc−1). For example, if Nproc=6, the HARQ process identifiers are assigned for 0 to 5, and if Nproc=12, the HARQ process identifiers are assigned for 0 to 11.
Moreover, when the UE is configured in the MIMO operation, the mapping relationship between the HARQ processes and the transport blocks is that when a HARQ process with a HARQ process identifier H is mapped to the primary transport block, a HARQ process with a HARQ process identifier given by ((H+8) mod 16) shall be mapped to the secondary transport block. For example, if the HARQ process with a HARQ process identifier 0 is mapped to the primary transport block, the HARQ process with a HARQ process identifier 8 is given to the secondary transport block.
Note that, when Nproc≦8, even if the UE is configured in the MIMO operation, the number of the HARQ processes that are used for receiving the secondary transport blocks is still 0. Take Nproc=6 for example and follow the mapping described above, the 6 HARQ processes with HARQ process identifiers 8 to 13 are mapped to the secondary transport blocks. However, all of the 6 HARQ process identifiers are assigned for 0 to 5, therefore the UE cannot configure the HARQ processes with HARQ process identifiers 8 to 13. In other words, even if the UE is configured in the MIMO operation, the UE cannot receive two transport blocks on the HS-PDSCH in one TTI when Nproc≦8.